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The concat operation joins multiple values into a single result. The return type is determined by the type of the first argument. If the first argument is a string, all subsequent arguments are converted to strings and concatenated into a single string. If the first argument is an array, the result is a new array containing all items from the input arrays and any non-array values. If the arguments are mixed, they are coerced according to the type of the first argument.

The syntax of the concat operation is:

[ $destination ] concat value1 value2 ...

The $destination is optional. If omitted, the result is assigned to the special slot _. At least one value must be provided.

When the first value is a string, all other values are stringified and appended in order:

$firstName let "Alice"
$lastName let "Johnson"
$fullName concat $firstName " " $lastName

This results in "Alice Johnson" stored in fullName.

When the first value is an array, all subsequent array values are unpacked and their items added in order. Any non-array value is inserted as a single item at its position:

$part1 let ["a", "b"]
$part2 let ["c", "d"]
$result concat $part1 "x" $part2

This results in ["a", "b", "x", "c", "d"] stored in result.

If the first argument is a string and any following argument is not a string, it is automatically stringified before being concatenated. If the first argument is an array and a following value is not an array, it is inserted as a single element. This coercion ensures predictable and useful behavior for common mixed-type patterns.

If only one argument is given, concat simply returns that argument unchanged. If no arguments are provided, the operation is invalid and results in an error.

The concat operation does not mutate any input values and produces a new value. All identifiers follow the single-assignment rule and must not be reused later in the same file.